1. Field of the Invention
The present invention relates to an image forming apparatus such as a laser copying machine or a laser printer including a revolver type developing unit, a toner cartridge arranged in the image forming apparatus, and a method for detecting a toner level of the toner cartridge.
2. Description of the Related Art
Usually, as image print processing in an image forming apparatus using an electrophotographic system such as a laser printer or a laser copying machine, steps of charging, exposure, development, transfer, and fixing are performed in this order. In the charging step, a photosensitive drum is charged. In the exposure step, an image is drawn on the photosensitive drum charged using a laser beam. An electric charge is removed from an area on the surface of the photosensitive drum where the laser beam is irradiated. In the development step, a toner is deposited on the photosensitive drum using electric charges remaining on the surface of the photosensitive drum. In the transfer step, the toner deposited on the photosensitive drum is transferred onto a sheet. In the fixing step, the toner transferred onto the sheet is hot-melted and fixed on the sheet.
As a device that performs the development step, a revolver type developing unit is known as disclosed in JP-A-2004-264539. The revolver type developing unit is used for color printing and has a structure in which plural toner cartridges are concentrically arranged inside of a revolver. For example, when there are three toner cartridges, toners of cyan, yellow, and magenta are stored in the respective toner cartridges.
Development processing using the revolver type developing unit is performed as described below. The toner cartridge of a first color (e.g., yellow) is brought close to a photosensitive drum to deposit the toner of the first color on the photosensitive drum. The toner deposited on the photosensitive drum is transferred onto a transfer belt. Subsequently, the revolver is rotated a fixed angle such that the toner cartridge storing the toner of a second color (e.g., magenta) comes close to the rotating drum to deposit the toner of the second color on the photosensitive drum. The toner of the second color deposited on the photosensitive drum in this way is transferred onto the transfer belt. The same processing is applied to the toner of a third color (e.g., cyan). The toners of the three colors transferred onto the transfer belt in this way are transferred onto a sheet.
In the development processing, in order to maintain a satisfactory quality of a print image, it is necessary to surely supply a required amount of a toner to the photosensitive drum. Thus, the image forming apparatus includes a mechanism for detecting a toner level of a toner cartridge and, when insufficiency of the toner occurs, urging a user to replace the toner cartridge or supply the toner to the toner cartridges.
As a method of detecting a toner level of a toner cartridge, there are known a method of detecting a toner level using a capacitance sensor disclosed in JP-A-2004-354904 and a method of detecting a toner level using an optical sensor including a light emitting device and a light receiving sensor disclosed in JP-A-7-225513 and JP-A-8-123187.
In the revolver type developing unit, the toner cartridges move according to the rotation of the revolver and the toners stored in the toner cartridges also move according to the movement of the toner cartridges. Therefore, in order to measure an accurate capacitance using the capacitance sensor, it is necessary to wait for the revolver to come to a standstill and the toners settling at the bottom of the toner cartridges come to a standstill. In the method of detecting the surface of a toner staying at the bottom of a toner cartridge using an optical sensor, it is impossible to measure an accurate toner level until the toner settles at the bottom of the toner cartridge. Therefore, it is difficult to measure a toner level on a real time basis.
In the conventional toner level detecting method using an optical sensor, in order to learn a toner level stepwise, it is necessary to provide plural sensors or provide a mechanism for moving a sensor. When plural sensors are provided, product cost increases. When a mechanism for moving a sensor is provided, an apparatus structure is complicated.